U.S. patent number 5,312,518 [Application Number 07/891,448] was granted by the patent office on 1994-05-17 for dry etching method.
This patent grant is currently assigned to Sony Corporation. Invention is credited to Shingo Kadomura.
United States Patent |
5,312,518 |
Kadomura |
May 17, 1994 |
Dry etching method
Abstract
A dry etching method whereby an SiO.sub.2 layer and an Si.sub.3
N.sub.4 layer may be etched with high selectivity for each other.
As etching gas, such sulfur fluorides as S.sub.2 F.sub.2 are used
which, when dissociated by electric discharges, will form
SF.sub.x.sup.+ as a main etchant for the SiO.sub.2 layer or F* as a
main etchant for the Si.sub.3 N.sub.4 layer and release sulfur in
the plasma. When the SiO.sub.2 layer is etched on the Si.sub.3
N.sub.4 layer as an underlying layer via a resist mask, nitrogen
atoms, removed from the underlying layer upon exposure thereof to
the plasma, will combine with sulfur in the plasma to form on the
exposed surface thereof such sulfur nitride compounds as
polythiazyl (SN).sub.x, which will, in turn, serve to achieve high
selectivity for the underlying layer. The SiO.sub.2 layer can also
be etched via an Si.sub.3 N.sub.4 mask patterned into a
predetermined shape, in which case sulfur nitride compounds formed
on the Si.sub.3 N.sub.4 mask will serve to achieve high selectivity
therefor.
Inventors: |
Kadomura; Shingo (Kanagawa,
JP) |
Assignee: |
Sony Corporation (Tokyo,
JP)
|
Family
ID: |
15606399 |
Appl.
No.: |
07/891,448 |
Filed: |
May 29, 1992 |
Foreign Application Priority Data
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May 31, 1991 [JP] |
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3-155454 |
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Current U.S.
Class: |
438/723;
257/E21.252; 257/E21.257; 257/E21.577; 438/591; 438/695 |
Current CPC
Class: |
H01L
21/31116 (20130101); H01L 21/76802 (20130101); H01L
21/31144 (20130101) |
Current International
Class: |
H01L
21/768 (20060101); H01L 21/02 (20060101); H01L
21/70 (20060101); H01L 21/311 (20060101); H01L
021/00 () |
Field of
Search: |
;156/643,646,662,652,653,657,655 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0265584 |
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May 1988 |
|
EP |
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0410635A1 |
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Jan 1991 |
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EP |
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63-141316 |
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Jun 1988 |
|
JP |
|
63-174322 |
|
Jul 1988 |
|
JP |
|
3060032 |
|
Mar 1991 |
|
JP |
|
3231426 |
|
Oct 1991 |
|
JP |
|
Other References
"D. C. Plasma Etching of Silicon By Sulfur Hexalfluoride: Mass
Spectrometric Study of the Discharge Products"; Wagner et al.;
1981; Plasma Chem. Plasma Process; 1(2); Abstract only. .
Plumb et al., "Gas-Phase Reactions of SF.sub.5, SF.sub.2, and SOF
with O(.sup.3 P): Their Significance in Plasma Processing", Plasma
Chemistry and Plasma Processing, vol. 6, No. 3, 1986, pp. 247-258.
.
Kushner, "A kinetic study of the plasma-etching process. I. A model
for the etching of Si and SiO.sub.2 in C.sub.n F.sub.m /H.sub.2 and
C.sub.n F.sub.m /O.sub.2 plasmas", Journal of Applied Physics, vol.
53, No. 4, Apr. 1982, pp. 2923-2938. .
Fior et al., "High-Selectivity, Silicon Dioxide Dry Etching
Process", Solid State Technology, vol. 31, No. 4, Apr. 1988, pp.
109-112. .
Bondur et al., "Selective Reactive Ion Etching of Silicon
Compounds", IBM Technical Disclosure Bulletin, vol. 21, No. 10,
Mar. 1979, p. 4015. .
Patent Abstracts of Japan, vol. 16, No. 307 (E-1229) Jul. 7, 1992
for Japanese Published Application No. 4084427 of Jul. 7, 1992.
.
Takuo Sugano, "Semiconductor Plasma Process Technology", pp.
133-134, published by Sangyo Tohio Company, Ltd. .
1980 National Convention Record, IEE Japan, vol. 5, paper
S6-2..
|
Primary Examiner: Hearn; Brian
Assistant Examiner: Goudreau; George
Attorney, Agent or Firm: Hill, Steadman & Simpson
Claims
What is claimed is:
1. A dry etching method for etching a silicon oxide based material
layer, comprising providing a silicon substrate, a three-layer gate
insulation film being formed on said silicon substrate, a gate
electrode formed on said gate insulation film, and an SiO.sub.2
layer covering said gate electrode with said three-layer gate
insulation film having a first SiO.sub.2 gate insulation film, a
Si.sub.3 N.sub.4 gate insulation film, and a second SiO.sub.2 gate
insulation film; creating a plasma etching gas containing at least
one compound selected from S.sub.2 F.sub.2, SF.sub.2, SF.sub.4 and
S.sub.2 F.sub.10 ; etching part of said SiO.sub.2 layer and part of
said second SiO.sub.2 gate insulation film to form a sidewall on
both sides of said gate electrode.
2. A dry etching method for etching a SiO.sub.2 layer formed on a
substrate and masked with a Si.sub.3 N.sub.4 material layer
patterned into a predetermined shape by providing a plasma etching
gas containing at least one compound selected from S.sub.2 F.sub.2,
SF.sub.2, SF.sub.4, and S.sub.2 F.sub.10, and by etching portion of
the SiO.sub.2 layer exposed by said Si.sub.3 N.sub.4 material
layer.
3. A dry etching method as claimed in claim 2 wherein said
substrate is composed of a silicon substrate, said SiO.sub.2 layer
is a SiO.sub.2 insulation film formed on said silicon substrate,
and the SI.sub.3 N.sub.4 mask is patterned into a predetermined
shape and formed on said SiO.sub.2 insulation film, said SiO.sub.2
insulation film being etched via said Si.sub.3 N.sub.4 mask to form
a connection hole in said SiO.sub.2 insulation film.
4. A dry etching method as claimed in claim 2 wherein said etching
gas contains at least one compound selected from H.sub.2, H.sub.2
S, and silane compounds as an additive gas capable of consuming
halogen radicals.
5. A dry etching method as claimed in claim 2 wherein said etching
gas contains rare gas as an additive gas.
6. A dry etching method comprising providing a substrate having a
Si.sub.3 N.sub.4 layer and a SiO.sub.2 layer on the Si.sub.3
N.sub.4 layer, creating a plasma etching gas containing a fluorine
gas selected from S.sub.2 F.sub.2, SF.sub.2, SF.sub.4 and S.sub.2
F.sub.10, maintaining the substrate at a temperature lower than
room temperature, etching the SiO.sub.2 layer while depositing a
sulfur material on the Si.sub.3 N.sub.4 layer, and subsequently
heating the substrate to remove the sulfur material.
7. A dry etching method according to claim 6, wherein said
substrate is composed of a silicon substrate, the Si.sub.3 N.sub.4
layer is a film formed directly on said silicon substrate, said
SiO.sub.2 layer being an insulation film formed on said Si.sub.3
N.sub.4 film, and a resist mask formed selectively on said
SiO.sub.2 insulation film, said step of etching causing the
SiO.sub.2 insulation film to be etched via said resist mask to form
a connection hole in said SiO.sub.2 layer insulation film to expose
a portion of the Si.sub.3 N.sub.4 film.
8. A dry etching method according to claim 6, wherein the plasma
etching gas contains an additive gas to consume halogen radicals,
said additive gas being at least one compound selected from
H.sub.2, H.sub.2 S and silane compounds.
9. A dry etching method as claimed in claim 6 wherein said etching
gas contains rare gas as additive gas.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dry etching method employed in
such applications as production of semiconductor devices. More
particularly, it relates to a dry etching method whereby a silicon
oxide based material layer and a silicon nitride based material
layer may be etched with high selectivity for each other.
2. Description of the Prior Art
The recent trend toward higher integration and performance of such
semiconductor devices as VLSIs and ULSIs requires dry etching
technologies for insulation films to achieve correspondingly higher
anisotropy, higher etchrate, higher selectivity, lower pollution,
and less damage with no compromise in these requirements.
Conventionally, etching gases typified by CHF.sub.3 gas, CF.sub.4
/H.sub.2 mixed gas, CF.sub.4 /O.sub.2 mixed gas, and C.sub.2
F.sub.6 /CHF.sub.3 mixed gas have been widely used to etch an
insulation film composed of silicon oxide (SiO.sub.x ;
particularly, x=2). All these etching gases are composed mainly of
fluorocarbon based gas whose molecule has C/F ratio (the ratio of
the numbers of carbon atoms to that of fluorine atoms in one
molecule) of 0.25 or higher. The common functions of these gases
include: (a) forming a C--O bond from a constituent element C on
the surface of a SiO.sub.2 layer and cleaving or weakening an Si--O
bond, (b) forming CF.sub.n.sup.+ (particularly, n=3) as a main
etchant for an SiO.sub.2 layer, and (c) generating relatively
carbon-rich plasma and thereby removing oxygen from SiO.sub.2 in
the form of CO or CO.sub.2 while achieving a lower etchrate and
higher selectivity for an underlying silicon layer as C, H, F, and
other constituent elements contribute to deposition of carbonaceous
polymers on the surface of the underlying layer.
It is to be noted that the above mentioned H.sub.2 and O.sub.2 are
used to regulate selectivity for an underlying silicon layer, in
other words, the apparent C/F ratio of an etching reaction system
by increasing or decreasing the quantity of F*.
Basically, etching gases for an SiO.sub.2 layer are also used to
etch an insulation layer composed of silicon nitride (Si.sub.x
N.sub.y particularly, x=3 and y=4). While the SiO.sub.2 layer is
etched mainly through an ion-assisted reaction, the Si.sub.x
N.sub.y layer is etched at a higher rate through a radical reaction
using F* as a main etchant. Such an etchrate difference is somewhat
predictable from the descending order in binding energy of an Si--F
bond (132 kcal/mole), Si--O bond (111 kcal/mole), and Si--N bond
(105 kcal/mole). Incidentally, these binding energy values are
cited from data shown in "Handbook of Chemistry and Physics" 69th
Edition (1988) edited by R. C. Weast (published by CRC Press Inc.
in Florida, U.S.A.) and may vary slightly according to any other
calculation method.
Meanwhile, the recent trend toward higher integration of
semiconductor devices requires correspondingly higher selectivity
for an SiO.sub.x layer and an Si.sub.x N.sub.y layer.
For instance, an Si.sub.x N.sub.y layer formed on an SiO.sub.x
layer as an underlying layer is etched in the LOCOS method where
the Si.sub.x N.sub.y layer is patterned to define an element
isolation region. This etching process requires extremely high
selectivity for the underlying layer now that a pad oxide film
(SiO.sub.2 layer) is decreased in thickness to minimize bird's beak
length.
On the other hand, an SiO.sub.x layer formed on an SiN.sub.x layer
as an underlying layer must be etched, for instance, in a contact
hole forming process. In recent years, a thin Si.sub.x N.sub.y
layer may be formed between the SiO.sub.x interlayer insulating
film and the Si substrate for the purpose of reducing damage to the
substrate in an over-etching process. To achieve this purpose, this
etching process also requires extremely high selectivity for the
underlying SiN.sub.x layer.
In principle, when achieving high selectivity for two laminated
dissimilar material layers, it is preferable that there should be a
great difference in interatomic bond energy between the chemical
bonds of the layers. In the case of an SiO.sub.x layer and an
Si.sub.x N.sub.y layer, in particular, it is essentially difficult
to achieve high selectivity between these two layers because of
little difference in bond energy between the bonds thereof (Si--O
bond and Si--N bond, respectively) and common bases of the etching
gas thereof. Therefore, persistent efforts have been made in
industrial sectors to develop technologies for overcoming such
difficulty.
In fact, some technologies have been reported for etching an
Si.sub.x N.sub.y layer formed on an SiO.sub.x layer with high
selectivity between these two layers.
For instance, the present inventor has disclosed in Japanese Patent
"KOKAI" 61-142744 (1986) one such technology whereby CH.sub.2
F.sub.2 or any other gas with a low C/F ratio are used as etching
gas with CO.sub.2 added at a molar ratio of 30 to 70%. More
specifically, such gases with a low C/F ratio will form
CF.sub.x.sup.+ (particularly, x=3) as an etchant for an SiO.sub.x
layer only by recombining with F*. The quantity of CF.sub.x.sup.+
thus formed will decrease when a great quantity of CO* is fed to
capture F* for removal from an etching reaction system in the form
of COF. As a result, an etchrate for the SiO.sub.x layer will also
decrease. Meanwhile, an Si.sub.x N.sub.y layer is etched by other
ions than CF.sub.x.sup.+ and radicals, so that the etchrate thereof
will remain almost unchanged even when a great quantity of CO.sub.2
is fed to the etching reaction system. Thus, high selectivity will
be achieved between the SiO.sub.x layer and the Si.sub.x N.sub.y
layer.
"Proceedings of Symposium on Dry Process", Vol. 88, No. 7, p. 86-94
(1987) has also reported another technology whereby an Si.sub.x
N.sub.y layer formed on an SiO.sub.x layer is etched by FCl which
will be formed in the gaseous phase by microwave discharge when
NF.sub.3 and Cl.sub.2 are fed to a chemical dry etching apparatus.
This technology is based on the fact that an Si--N bond with a 30%
ionicity has a stronger covalency than an Si--O bond with a 55%
ionicity. Namely, the Si.sub.x N.sub.y layer, whose chemical bond
(Si--N bond) is similar in nature to the chemical bond (covalent
bond) of single-crystal silicon, will be etched by F*, Cl*, and
other radicals resulting from dissociation of FCl while the
SiO.sub.x layer will almost never be etched by these radicals.
Thus, high selectivity will also be achieved between the two
layers.
As mentioned above, some technologies have been reported for
selectively etching an Si.sub.x N.sub.y layer formed on an
SiO.sub.x layer. This is a natural consequence considering the
difference in etchrate between the two layers. When the Si.sub.x
N.sub.y layer is etched mainly through a radical reaction, exposure
of the SiO.sub.x layer to a plasma in the etching process will
inevitably result in reduction of the etchrate.
However, there are some difficulties with the conventional
technologies. For instance, anisotropic etching is essentially
difficult with the above-mentioned technology using FCl because it
is based on a radical reaction.
Conversely, no technology has been reported for selectively etching
an SiO.sub.x layer formed on an Si.sub.x N.sub.y layer. This
etching process etches the SiO.sub.x layer mainly through an
ion-assisted reaction and involves even greater difficulty in
achieving high selectivity between the two layers because radicals
formed invariably in the etching reaction system will increase the
etchrate upon exposure of the Si.sub.x N.sub.y layer to the plasma.
In fact, it is certain that this etching process will be needed in
the future and it is required to realize it as soon as
possible.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a dry
etching method whereby an SiO.sub.x layer and an Si.sub.x N.sub.y
layer may be etched with high selectivity for each other.
According to one aspect of the present invention, it provides a
process of etching an SiO.sub.x layer formed on an Si.sub.x N.sub.y
layer with high selectivity between these two layers.
This process uses four sulfur fluorides S.sub.2 F.sub.2, SF.sub.2,
SF.sub.4, and S.sub.2 F.sub.10, which have been proposed by the
present inventor as effective chlorofluorocarbon-free (CFC-free)
etching gas for an SiO.sub.x layer. These sulfur fluorides will be
dissociated by electric discharges to form SF.sub.x.sup.+ in the
plasma, which will, in turn, act as a main etchant for an SiO.sub.x
layer. Si will be removed from the SiO.sub.x layer in the form of
SiF.sub.x.
The four sulfur fluorides have an important advantage over the
conventionally well-known sulfur fluoride SF.sub.6 in that they
have a high S/F ratio (the ratio of the numbers of sulfur atoms to
that of fluorine atoms in one molecule) and form free sulfur in the
plasma. The sulfur thus formed will adsorb on the surface of a
target substrate (wafer) maintained at a lower temperature than
room temperature. Likewise, sulfur will adsorb on the surface of a
target layer (SiO.sub.2 layer) and form SO.sub.x (x=2 or 3) with
the assistance of high-energy incident ions striking on that
surface for removal from the etching reaction system. Meanwhile,
sulfur will be deposited on, and competitively removed through
sputtering from, the surface of a resist mask and an underlying
silicon layer, thus improving selectivity for both the resist mask
and the underlying layer. Further, the sulfur will serve to protect
pattern sidewalls, on which no incident ion will strike in
principle.
The sulfur deposits will sublime immediately upon heating of the
substrate after etching and therefore avoid the danger of particle
pollution.
When the SiO.sub.x layer has almost been etched, the underlying
Si.sub.x N.sub.y layer will be exposed to the plasma while F*
present in the plasma will extract silicon atoms from the surface
of the Si.sub.x N.sub.y layer to form dangling nitrogen bonds. The
dangling nitrogen bonds present on the surface of the Si.sub.x
N.sub.y layer has already been discussed in "Semiconductor Plasma
Process Technology" by Takuo Sugano, P. 133-134 (published by
Sangyo Tosho Co., Ltd.), and 1980 National Convention Record, IEE
Japan, Vol. 5, S 6-2. According to the present invention, the
dangling nitrogen bonds will combine with the sulfur formed in the
plasma to form various sulfur nitride based compounds, which, in
turn, will serve to protect the surface of the Si.sub.x N.sub.y
layer and achieve high selectivity for this layer.
The most typical of the above-mentioned sulfur nitride based
compounds is polythiazyl represented by a general formula
(SN).sub.x. The simplest process of forming polythiazyl is that the
sulfur formed in the plasma combines with the dangling nitrogen
bonds to form thiazyl (N.tbd.S), which has unpaired electrons
analogous to those of an oxygen analog nitrogen oxide (NO) and
polymerizes easily to form (SN).sub.2, (SN).sub.4, and (SN).sub.x.
(SN).sub.2 polymerizes easily at temperatures around 20.degree. C.
to form (SN).sub.4 and (SN).sub.x, and decomposes at temperatures
around 30.degree. C. (SN).sub.4 is a ring compound with a melting
point of 178.degree. C. and a decomposition point of 206.degree.
C.
The property and structure of (SN).sub.x are described in detail in
J. Am. Chem. Soc., Vol. 29, p. 6358-6363 (1975). (SN).sub.x is a
polymerized compound that will remain stable up to temperatures
around 208.degree. C. under atmospheric pressure and 140.degree. to
150.degree. C. under reduced pressure. In a crystalline state, it
is so structured that principal chains each composed of repetitive
covalent bonds S--N--S--N-- - - - are oriented in parallel to one
another. Consequently, when deposited on a layer, sulfur nitride
based compounds, typically (SN).sub.x, will effectively inhibit F*
and other radicals from invading the layer. Even when accelerated
incident ions strike the layer, they will absorb or alleviate the
impact of the ions on the layer by producing so-called sponge
effects derived from changes in their bond angle, conformation, and
other property.
Besides sulfur, F* deriving from sulfur fluoride is also present in
the plasma, so that the resulting fluorine may combine with the
(SN).sub.x to form thiazyl fluoride. Further, when nitrogen based
gas is added to regulate the quantity of F* formed in the plasma,
the resulting hydrogen may combine with the (SN).sub.x to form
hydrogen thiazyl.
Under some conditions, the above-mentioned sulfur nitride based
compounds may be S.sub.4 N.sub.2 (melting point: 23.degree. C.),
S.sub.11 N.sub.2 (melting point: 150.degree.-155.degree. C.),
S.sub.15 N.sub.2 (melting point: 137.degree. C.), S.sub.16 N.sub.2
(melting point: 122.degree. C.), and other cyclic sulfur nitride
compounds containing much more sulfur atoms than nitrogen atoms, as
well as S.sub.7 NH (melting point: 113.5.degree. C.), 1, 3--S.sub.6
(NH).sub.2 (melting point: 130.degree. C.), 1, 4--S.sub.6
(NH).sub.2 (melting point: 133.degree. C.), 1, 5--S.sub.6
(NH).sub.2 (melting point: 155.degree. C.), 1, 3, 5--S.sub.5
(NH).sub.3 (melting point: 124.degree. C.), 1, 3, 6--S.sub.5
(NH).sub.3 (melting point: 131.degree. C.), S.sub.4 (NH).sub.4
(melting point: 145.degree. C.), and other imide compounds composed
of hydrogen atoms combined with nitrogen atoms of the
abovementioned cyclic sulfur nitride compounds.
All these sulfur nitride based compounds with sulfur and nitrogen
as the constituent elements thereof will be removed in the form of
N.sub.2, NO.sub.x, SO.sub.x, etc. upon removal of the resist mask
through O.sub.2 plasma aching without remaining on the wafer or
causing particle pollution.
According to another aspect of the present invention, it provides a
process of etching an SiO.sub.x layer masked with an Si.sub.x
N.sub.y layer while achieving high selectivity for the Si.sub.x
N.sub.y mask.
This process achieves high selectivity on the same principle as
mentioned above. In this process, the top surface of the Si.sub.x
N.sub.y mask is exposed to the plasma at the start of etching,
whereupon F* present in the plasma will remove silicon atoms from
the surface of the Si.sub.x N.sub.y mask to form dangling nitrogen
bonds and sulfur will combine with the dangling nitrogen bonds to
form various sulfur nitride based compounds, typically (SN).sub.x,
which will, in turn, cover the surface of the Si.sub.x N.sub.y
mask. Meanwhile, the SiO.sub.x layer will be etched at a high rate
while free sulfur in the plasma will serve to protect the pattern
sidewalls.
Conventionally, the SiO.sub.x layer used to be etched by
high-energy incident ions and often cause dimension losses due to
recession of the Si.sub.x N.sub.y mask. This problem can be solved
by the present invention.
In this process, the Si.sub.x N.sub.y layer formed on the target
SiO.sub.x layer and covered with a resist mask must be etched to
form the Si.sub.x N.sub.y mask. Such preliminary etching can be
performed with high selectivity in the above-mentioned process.
According to the present invention, once the Si.sub.x N.sub.y mask
is formed, the resist mask is removed from the Si.sub.x N.sub.y
layer and the SiO.sub.x layer is etched with the surface thereof
covered only with the Si.sub.x N.sub.y mask. As a result, there is
no danger of carbonaceous decomposition products being formed in
the etching reaction system due to decomposition of the resist
material and hence the advantage of reducing particle
pollution.
As is clear from the foregoing description, the present invention
provides a dry etching method whereby an SiO.sub.x layer and
Si.sub.x N.sub.y layer may be etched with high selectivity for each
other. More particularly, the present invention realizes an
unprecedented process of selectively etching an SiO.sub.x layer
formed on an Si.sub.x N.sub.y layer, thus opening up the
possibility of developing novel semiconductor device structures to
say nothing of providing an effective CFC-free etching method.
Accordingly, the present invention is particularly useful for such
industrial applications as production of large-scale integrated
high-performance semiconductor devices conforming to strict design
rules.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a-1d are schematic cross-sectional views showing the stages
of a dry etching method applied in one preferred embodiment to a
contact hole forming process. FIG. 1a shows a stage at which a
resist mask was formed on an SiO.sub.2 inter-layer insulation film.
FIG. 1b shows a stage at which the SiO.sub.2 inter-layer insulation
film was etched. FIG. 1c shows a stage at which a resist mask and
sulfur and (SN).sub.x deposits were removed through plasma aching.
FIG. 1d shows a stage at which an Si.sub.3 N.sub.4 base film was
selectively removed from a contact hole.
FIGS. 2a-2c are schematic cross-sectional views showing the stages
of a dry etching method applied in another preferred embodiment to
an etch-back process in which a sidewall is formed on both sides of
a gate electrode formed on a gate insulation film having a
so-called ONO structure. FIG. 2a shows the stage at which an
SIO.sub.2 layer was formed all over a wafer. FIG. 2b shows the
stage at which the SiO.sub.2 layer or a second SiO.sub.2 insulation
film was etched back or selectively etched, respectively. FIG. 2c
shows the stage at which sulfur and (SN).sub.x deposits were
decomposed or sublimed for removal through heating.
FIGS. 3a-3c are schematic cross-sectional views showing the stages
of a dry etching method applied in still another embodiment to a
contact hole forming process. FIG. 3a shows the stage at which an
Si.sub.3 N.sub.4 mask was formed on an SiO.sub.2 inter-layer
insulation film before it was etched. FIG. 3b shows the stage at
which the SiO.sub.2 inter-layer insulation film was etched. FIG. 3c
shows the stage at which sulfur and (SN).sub.x deposits were
decomposed or sublimed for removal through heating.
DETAILED DESCRIPTION OF THE INVENTION
The following paragraphs describe some preferred embodiments of the
present invention.
EXAMPLE 1
In the first example, a dry etching method according to one aspect
of the present invention is applied to a contact hole forming
process in which an SiO.sub.2 inter-layer insulation film is etched
by S.sub.2 F.sub.2 /H.sub.2 mixed gas. This process is described by
referring to FIGS. 1a-1d. It is to be noted that FIGS. 1a-1d are
drawn with a smaller aspect ratio than usual in order to provide
schematic representations.
Referring first to FIG. 1a, a silicon substrate (wafer) 1 carrying
an impurity-diffused region 2 was provided on the surface thereof
with an Si.sub.3 N.sub.4 base film 3 which was formed in the
thickness of 10 nm, for instance, by low pressure chemical vapor
deposition (LPCVD). Then, the Si.sub.3 N.sub.4 base film 3 was
provided on the surface thereof with a SiO.sub.2 inter-layer
insulation film 4 which was formed in the thickness of 500 nm by
normal pressure CVD. Further, the SiO.sub.2 inter-layer insulation
film 4 was coated on the surface thereof with chemical
amplification negative type three-component photoresist SAL-601
(brand name of Shipley Co., Inc.) and a resist mask 5 was so formed
as to have an opening 5a by excimer laser lithography and alkaline
development.
The target wafer thus formed was set on a wafer supporting
electrode provided on a magnetron reactive ion etching (RIE)
apparatus and cooled to about -50.degree. C. by an ethanol
refrigerant which was fed from a cooling system, for instance, a
chiller for circulation through a cooling pipe housed in the wafer
supporting electrode. In this state, the SiO.sub.2 inter-layer
insulation film 4 was etched, for instance, with an S.sub.2 F.sub.2
flow rate of 50 SCCM, H.sub.2 flow rate of 20 SCCM, gas pressure of
1.3 Pa (10 mTorr), and RF power of 1000 W (2 MHz).
Referring next to FIG. 1b, there is schematically shown a mechanism
of the above etching. In the drawing, the composition formulas
encircled in a dotted line represent those chemical species which
are deposited on, and simultaneously removed through sputtering
from, the surface of the wafer while encircled in a continuous line
are those chemical species which remain deposited on the surface of
the wafer.
The SiO.sub.2 inter-layer insulation film 4 was etched by S*, F*,
and other radicals formed in the plasma as the reaction of these
radicals was assisted by SF.sub.x.sup.+, S.sup.+, and other ions.
Also present in the plasma was free sulfur, which resulted from
dissociation of S.sub.2 F.sub.2 by electric discharges and adsorbed
on the surface of the wafer as it was cooled to lower temperature
than room temperature. Likewise, sulfur adsorbed on the surface of
the SiO.sub.2 inter-layer insulation film 4 and combined with
oxygen fed from that surface through sputtering to form SO.sub.x
for removal from the etching reaction system. Thus, the sulfur
present on the surface of the SiO.sub.2 inter-layer insulation film
4 never reduced the etchrate thereof. By contrast, sulfur was
deposited on, and simultaneously removed through sputtering from,
the surface of the resist mask 5, thus reducing the etchrate
thereof and improving selectivity for the resist mask 5. Meanwhile,
sulfur deposited on pattern sidewalls, on which no incident ion
struck in principle, and served to protect it, forming a contact
hole 4a having a vertical wall.
It is to be noted that H.sub.2 is used as an additive gas to
increase the apparent S/F ratio of the etching reaction system and
prevent F* from deteriorating selectivity for the base layer. More
specifically, H.sub.2 will be dissociated to form H*, which is
capable of capturing part of F* for removal from the etching
reaction system in the form of HF. Such S/F ratio regulation is
extremely useful in preventing deterioration of anisotropy or
selectivity for the underlying layer in an over-etching process
where F* will become excessive.
Referring further to FIG. 1b, when the SiO.sub.2 inter-layer
insulation film 4 was almost etched, the Si.sub.3 N.sub.4 base film
3 was exposed at the bottom of the contact hole 4a, whereupon
nitrogen supplied from the Si.sub.3 N.sub.4 base film 3 combined
with sulfur present in the plasma to form a sulfur nitride based
compound, that is, a polymerized thiazyl compound (SN).sub.x in the
drawing. (SN).sub.x thus formed deposited on the Si.sub.3 N.sub.4
base film 3, reducing the etchrate thereof and achieving
selectivity ratio of about 15 for the Si.sub.3 N.sub.4 base film
3.
Referring next to FIG. 1c, when the wafer was set on a plasma asher
to remove the resist mask 5 in O.sub.2 plasma, the sulfur deposited
on the pattern sidewalls was burned for removal in the form of
SO.sub.x while the (SN).sub.x deposited on the surface of the
SiO.sub.2 inter-layer insulation film 4 were also burned or
decomposed for removal in the form of N.sub.2, NO.sub.x, SO.sub.x,
etc.
Referring finally to FIG. 1d, when the wafer was immersed in hot
aqueous phosphate solution, the Si.sub.3 N.sub.4 base film 3
exposed at the bottom of the contact hole 4a was decomposed for
removal.
In the above example, the contact hole 4a was formed in an
excellent anisotropic shape without causing damage to the
impurity-diffused region 2 or particle pollution.
EXAMPLE 2
In the second example, a dry etching method according to one aspect
of the present invention is applied to a contact hole forming
process in which an SiO.sub.2 inter-layer insulation film is etched
by S.sub.2 F.sub.2 /H.sub.2 S mixed gas.
In the second example, the same wafer as in the first example is
used as an etching sample.
The wafer was set on a wafer supporting electrode provided on an RF
biased magnetically enhanced microwave plasma etching apparatus. In
this state, the SiO.sub.2 inter-layer insulation film 4 was etched,
for instance, with an S.sub.2 F.sub.2 flow rate of 50 SCCM, H.sub.2
flow rate of 15 SCCM, gas pressure of 1.3 Pa (10 mTorr), microwave
power of 850 W, RF bias power of 200 W (400 kHz), and wafer
temperature of 50.degree. C.
In the second example, etching proceeded by the same mechanism as
in the first example, except that H.sub.2 S was used as additive
gas to supply sulfur, thereby promoting sulfur deposition.
In the second example, too, the contact hole 4a was formed in an
excellent anisotropic shape while achieving high selectivity for
the Si.sub.3 N.sub.4 base film 3.
EXAMPLE 3
In the third example, a dry etching method according to one aspect
of the present invention is applied to an etch-back process in
which a sidewall composed of an SiO.sub.2 layer is formed on both
sides of a gate electrode formed on a gate insulation film
containing an Si.sub.3 N.sub.4 film. This process is classified
among manufacturing processes for MOS-FET having an LDD structure.
This process is described by referring to FIGS. 2a-2c.
Referring first to FIG. 2a, a silicon substrate 11 was provided on
the surface thereof with a gate insulation film 16 having a
so-called ONO (oxide film/nitride film/oxide film) structure. Then,
the gate insulation film 16 was provided on the surface thereof
with a gate electrode 17 which was formed through patterning from
an n.sup.+ type polysilicon layer. Masked with the gate electrode
17, the silicon substrate 11 was implanted with ions to form
low-density-impurity-diffused regions 12 on the surface thereof.
Further, an SiO.sub.2 layer 18 was deposited all over the wafer by
CVD. The gate insulation film 16 was composed of a first SiO.sub.2
gate insulation film 13 (4 nm thick), Si.sub.3 N.sub.4 gate
insulation film 14 (6 nm thick), and second SiO.sub.2 gate
insulation film 15 (2 nm thick) which were laminated in this order
on the silicon substrate 11.
The wafer was set on a wafer supporting electrode provided on an RF
biased magnetically enhanced microwave plasma etching apparatus. In
this state, the SiO.sub.2 layer 18 was etched back and the second
SiO.sub.2 gate insulation film 15 was etched, for instance, with an
S.sub.2 F.sub.2 flow rate of 50 SCCM, gas pressure of 1.3 Pa (10
mTorr), RF power of 200 W (400 kHz), and wafer temperature of
-80.degree. C.
In the third example, etching proceeded by the same mechanism as in
the first example, except that neither H.sub.2 nor H.sub.2 S was
used as additive gas to consume F* and that the wafer was cooled to
even a lower temperature, thereby inhibiting reaction by F* and
promoting sulfur deposition.
Referring next to FIG. 2b, when etching was completed, the gate
electrode 17 and the Si.sub.3 N.sub.4 gate insulation film 14 were
exposed to the plasma, whereupon sidewalls 18a were formed on both
sides of the gate electrode 17. Unlike the SiO.sub.2 layer 18, the
gate electrode 17 ceased to be supplied with oxygen and had sulfur
depositing on the surface thereof, thus greatly reducing the
etchrate and achieving high selectivity therefor. Meanwhile, the
Si.sub.3 N.sub.4 gate insulation film 14 also had sulfur nitride
based compounds, typically (SN).sub.x, depositing on the surface
thereof, thus greatly reducing the etchrate and achieving a
selectivity ratio of about 20 therefor.
Referring finally to FIG. 2c, when the wafer was heated to about
130.degree. C. or higher after completion of the etching, the
sulfur and sulfur nitride based compound were removed from the
surface of the wafer immediately. Namely, the sulfur was sublimed
upon heating of the wafer to temperatures around 90.degree. C. and
the sulfur nitride based compound was then decomposed for removal
upon further heating of the wafer. In the third example, the
sidewalls 18a were formed without causing damage to the underlying
low-density-impurity-diffused regions 12.
EXAMPLE 4
In the fourth example, a dry etching method according to another
aspect of the present invention is applied to a contact hole
forming process in which an SiO.sub.2 inter-layer insulation film
was etched by S.sub.2 F.sub.2 /H.sub.2 mixed gas via an Si.sub.3
N.sub.4 mask. This process is described by referring to FIGS.
3a-3c.
Referring first to FIG. 3a, a silicon substrate (wafer) 21 carrying
an impurity-diffused region 22 was provided on the surface thereof
with an SiO.sub.2 inter-layer insulation film 23 which was formed
to a thickness of 1 .mu.m, for instance, by normal pressure CVD.
Further, the SiO.sub.2 inter-layer insulation film 23 was provided
on the surface thereof with Si.sub.3 N.sub.4 masks 24 patterned
into a predetermined shape. The Si.sub.3 N.sub.4 masks 24 were so
formed as to have an opening 24a by etching an Si.sub.3 N.sub.4
layer (100 nm thick) formed, for instance, by low pressure CVD via
a chemical amplification photoresist mask formed by excimer laser
lithography and alkaline development. The photoresist mask had been
removed from the Si.sub.3 N.sub.4 layer through ashing.
The target wafer thus formed was set on a wafer supporting
electrode provided on an RF biased magnetically enhanced microwave
plasma etching apparatus. In this state, the SiO.sub.2 inter-layer
insulation film 23 was etched, for instance, with an S.sub.2
F.sub.2 flow rate of 50 SCCM, H.sub.2 flow rate of 20 SCCM, gas
pressure of 1.3 Pa (10 mTorr), microwave power of 850 W, and RF
bias power of 200 W (400 kHz).
Referring next to FIG. 3b, when etching was started, the Si.sub.3
N.sub.4 masks 24, which has already been exposed to the plasma,
were coated immediately on the surface thereof with sulfur nitride
based compounds, typically (SN).sub.x, thus greatly reducing the
etchrate and achieving an selectivity ratio of about 20 therefor.
As a result, the SiO.sub.2 inter-layer insulation film 23 was
etched without causing dimension losses due to recession of the
Si.sub.3 N.sub.4 mask while a contact hole 23a was formed without
deteriorating the sectional form thereof.
Meanwhile, the SiO.sub.2 inter-layer insulation film 23 was etched
into an anisotropic shape as sulfur was deposited on the sidewalls
thereof and served to protect them. Upon exposure of the underlying
impurity-diffused region 22 to the plasma, sulfur also was
deposited on the surface thereof, thus greatly reducing the
etchrate, and achieving high selectivity in an over-etching process
therefor.
Referring finally to FIG. 3c, when etching was completed, the wafer
was heated to about 130.degree. C. or higher to remove the sulfur
and sulfur nitride based compound.
Conventionally, an SiO.sub.2 layer used to be etched by high energy
incident ions, which, when sputtering on a resist mask, will cause
recession thereof and resulting particle pollution. This problem
can be solved by a dry etching method according to another aspect
of the present invention, whereby the SiO.sub.2 layer may be etched
via an Si.sub.3 N.sub.4 layer instead of the resist mask. The
Si.sub.3 N.sub.4 layer thus substituting for the resist mask may be
left unremoved as part of an insulation film.
While the present invention has been described in four preferred
examples thereof, it is to be understood that the present invention
is not limited to those examples and that various changes and
modifications may be made in the present invention without
departing from the spirit and scope thereof. For instance, etching
gas may contain any additive gas other than H.sub.2 and H.sub.2 S,
such as silane based gas for increasing the S/F ratio of gases in
an etching reaction system and He, Ar, and other rare gases for
producing sputtering, cooling, and dilution effects.
Further, etching will proceed by the same mechanism as in the
preferred examples even when any sulfur fluoride proposed herein
other than S.sub.2 F.sub.2 is used as etching gas.
* * * * *